Device for backscatter communication

ABSTRACT

Backscatter communication includes receiving electromagnetic energy from a base station and encoding first data and second data. The first data is encoded at a first frequency by adjusting a radar cross-section of a device to modulate the electromagnetic energy reflected back to the base station. The second data is encoded at a second frequency by limiting the adjusting of the plurality of radar cross-sections to either a first subset or a second subset of the plurality of radar cross-sections for a length of time. The second frequency is lower than the first frequency.

REFERENCE TO RELATED APPLICATIONS

The present application is related to a U.S. application Ser. No.14/502,175 entitled “Receiver for Backscatter Communication,” filed onthe same day, Sep. 30, 2014.

TECHNICAL FIELD

This disclosure relates generally to backscatter communication, and inparticular but not exclusively, relates to radio-frequencyidentification (“RFID”) tags.

BACKGROUND INFORMATION

Radio-frequency identification (“RFID”) communication is one example ofbackscatter communication. RFID communication generally includes a “basestation transceiver” that broadcasts/transmits electromagnetic energyand then interprets data from reflections of the broadcastedelectromagnetic energy. A “tag” reflects a portion of theelectromagnetic energy back to the base station in order to communicatedata to the reader. To encode data (e.g. an identification number) inthe reflected portion, a passive (battery-free) tag may harvest powerfrom the broadcasted electromagnetic energy and use the harvested powerto modulate the electromagnetic energy reflected back to the basestation. In contrast, a battery-powered tag uses a battery to powercircuitry that modulates the electromagnetic energy reflected back tothe base station. Passive tags generally have a range that is muchshorter than battery powered tags.

Backscatter communication (including RFID communication systems) isincreasingly important as the tags can be manufactured relatively smalland RFID communication doesn't require line-of-site between the basestation and the tag. As RFID communication systems become moreprevalent, demand has increased for sending larger amounts ofinformation in shorter periods of time using backscatter communication.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 illustrates a backscatter communication system including a basestation and tags, in accordance with an embodiment of the disclosure.

FIG. 2A is a functional block diagram illustrating a base station forfacilitating backscatter communication, in accordance with an embodimentof the disclosure.

FIG. 2B is a functional block diagram illustrating example backscatterreceiving circuitry, in accordance with an embodiment of the disclosure.

FIG. 3A illustrates a block diagram of a device that includes an exampletag, in accordance with an embodiment of the disclosure.

FIG. 3B illustrates a block diagram of a device that includes an exampletag, in accordance with an embodiment of the disclosure.

FIG. 4A illustrates a chart showing the voltage of a signal over time,in accordance with an embodiment of the disclosure.

FIG. 4B illustrates a zoomed-in portion of the chart in FIG. 4A, inaccordance with an embodiment of the disclosure.

FIG. 5 illustrates a chart showing vector radar cross sections over timeof an antenna, in accordance with an embodiment of the disclosure.

FIG. 6 illustrates a flow chart illustrating a tag-side method ofbackscatter communication, in accordance with an embodiment of thedisclosure.

FIG. 7 illustrates a flow chart illustrating a method of backscattercommunication utilizing a base station, in accordance with an embodimentof the disclosure.

DETAILED DESCRIPTION

Embodiments of a system and method for backscatter communications aredescribed herein. In the following description, numerous specificdetails are set forth to provide a thorough understanding of theembodiments. One skilled in the relevant art will recognize, however,that the techniques described herein can be practiced without one ormore of the specific details, or with other methods, components,materials, etc. In other instances, well-known structures, materials, oroperations are not shown or described in detail to avoid obscuringcertain aspects.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of the phrases “in one embodiment” or “in an embodiment” invarious places throughout this specification are not necessarily allreferring to the same embodiment. Furthermore, the particular features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

FIG. 1 illustrates a backscatter communication system 100 that includesa base station transceiver 103 and tags included in mobile devices 101,in accordance with an embodiment of the disclosure. Backscattercommunication system 100 uses backscatter communications to provide ashort range (e.g., up to 20 m), high bandwidth (e.g., 20 to 100 Mbps),and low power (e.g., less than 1 mW) wireless communication link todeliver data from one or more mobile devices 101 to base station 103.One example of backscatter communication is commonly known asRadio-Frequency Identification (“RFID”). RFID is often used towirelessly communicate an identification code of an object (e.g. keycard, consumer product). However, backscatter communications includingRFID can also be used to stream data sets that are larger than mereidentification codes/numbers

The backscatter communication link is achieved by integratingbackscatter tags (e.g., semi-passive tags) into mobile devices 101. Thedesign leverages asymmetric power budgets between wired base station 103and mobile devices 101 to provide a low power solution on the mobiledevice side by relying upon the readily available power on the basestation side.

Base station 103 includes one or more antennas that broadcastelectromagnetic (“EM”) energy 104 towards mobile devices 101 and receivemodulated backscatter reflections 105 of EM energy 104. Modulatedbackscatter reflections 105 are referred to as the backscatter signal orbackscatter channel. The backscatter tags integrated into mobile devices101 do not transmit any RF or microwave power. Rather, they operate bymodulating the reflections of EM energy 104. The backscatter reflectionsare encoded with the data by modulating the radar signatures or radarcross-section of mobile devices 101 with data and base station 103demodulates the received radar signatures reflected from mobile devices101 to extract the embedded data. One technique for modulating the radarcross-section of mobile devices 101 is to modulate an impedance loadcoupled to the backscatter antenna on mobile device 101. This impedancemodulation is a low power task when compared to an active transmittersuch as WiFi or Bluetooth radio.

Some Radio-Frequency Identification (“RFID”) tags are fully passivedevices that include no independent power source and harvest theirenergy for operation from EM energy 104. However, energy harvesting fromEM energy 104 effectively slows the data rate of the backscatterchannel, since the backscatter antenna will typically be optimized forharvesting power and not necessarily improving the signal-to-noise ratio(“SNR”) of the backscatter channel. Additionally, fully passive RFIDtags often pause for periodic power harvesting, which interrupts ordelays data transmission. Energy harvesting reduces the read range forbase station 103 because more incident EM radiation 104 is required topower up a backscatter tag than is required for the backscattercommunications alone. Conventional fully passive backscatter tags employslower data rates, as energy consumption on the backscatter tag ishighly dependent on clock speed.

Embodiments of the backscatter tags embedded within mobile devices 101may be partially passive devices, which do not harvest energy from EMradiation 104. Rather, the backscatter tags are powered by the mainbattery of mobile devices 101. Since modulating the impedance loadrequires a modest power budget (e.g., 15 uW), the backscattertransmission does not impact battery life in a significant manner.Additionally, by not harvesting power from EM energy 104, thebackscatter antennas and modulation load impedances can be optimized forreflecting EM energy 104 to improve SNR, reduce bit rate errors, andincrease data throughput of the backscatter channel. By not harvestingpower from EM energy 104 to power the backscatter tag, some embodimentsdisclosed herein can operate with higher clock rates and greater datathroughput. Other embodiments of the disclosure may harvest energy fromEM energy 104.

EM energy 104 may be broadcast using a variety of different carrierfrequencies. For example, EM energy 104 may operate on unencumberedfrequencies such as 915 MHz, 2.45 GHz, 5.8 GHz, and 61.25 GHz. Thebackscatter tags may modulate the backscatter signal using a variety oftechniques and symbol constellations for encoding the data onto thebackscatter channel. For example, binary phase shift keying (“BPSK”) orbinary amplitude shift keying (“BASK”) may be used. To achieve higherdata rates, quadrature amplitude modulation (“QAM”) may be used tomodulate the load impedances applied to the backscatter antenna tochange the vector radar cross section (“RCS”) of the antenna. Usinghigher carrier frequencies and larger QAM constellations (e.g., 16-QAM,64-QAM, etc.) can achieve higher data rates (e.g., 100 Mbps). In someembodiments, the symbol constellation for encoding data on thebackscatter channel can be adaptively updated based upon the environment(e.g., noise, multi-path reflections, etc.) to improve throughput,improve SNR, or make the backscatter link less susceptible todegradation as a mobile device 101 moves through their environments.

Mobile devices 101 represent a variety of different devices, includingmobile phones 101A, head wearable displays 101B, smart wrist watches101C, tablets, laptops, body-mountable devices, body implantables, orother mobile devices operating with limited power budgets. Embodimentsdisclosed herein provide a backscatter channel having sufficientbandwidth to wirelessly stream data (e.g. video data, audio data, textdata) from the mobile devices 101 to base station 103. Base station 103may then transfer the streamed data via a wired (e.g. Ethernet) orwireless (e.g. WiFi) connection to other devices such as televisions,servers, or other mobile devices. In the illustrated embodiment, basestation 103 is a standalone box. In other embodiments, base station 103may be integrated into a television, home computer, computer monitor,WiFi access point, cable modem, harddrive, router, set-top box, or otherelectronic device. In an embodiment where base station 103 is, or isincluded in a WiFi access point, EM energy 104 could be the WiFitransmission and tags could reflect EM energy 104 back to base station103.

FIG. 2 is a functional block diagram illustrating an example basestation 203 for facilitating backscatter communication, in accordancewith an embodiment of the disclosure. Base station 203 is one possibleimplementation of base station 103 illustrated in FIG. 1. Theillustrated embodiment of base station 203 includes a backscattertransceiver 205, backscatter antennas 210 and 215, control circuitry220, wired interface(s) 230, a power regulator 235, and one or morewireless communication antenna(s) 242. The illustrated embodiment ofbackscatter transceiver 205 includes backscatter transmit circuitry 245and backscatter receive circuitry 250. The illustrated embodiment ofcontrol circuitry 220 includes logic 287. FIG. 2 illustrates functionalcomponents of base station 203 and not necessarily a structural layout.It should be appreciated that the components of base station 203 may beimplemented entirely in hardware, entirely in software/firmware, orusing a hybrid of both software/firmware and hardware.

Backscatter transceiver 205 is the communication channel for deliveringhigh bandwidth data from mobile devices 101 to base station 203. In oneembodiment, the upstream direction from backscatter transmit circuitry245 is a non-communicative path, but merely outputs EM energy 212 as asort of radar signal. In other embodiments, backscatter transmitcircuitry 245 can modulate data onto EM energy 212 to provide anupstream broadcast data path to mobile devices 101. Backscatter transmitcircuitry 245 can output EM energy 212 having a variety of differentfrequencies such as 915 MHz, 2.45 GHz, 5.8 GHz, 61.25 GHz, or otherwise.Backscatter receive circuitry 250 implements the downstream path frommobile devices 101 and operates by demodulating the backscatter signalreflected by mobile devices 101. In essence, backscatter receivecircuitry 250 is demodulating the received radar signature reflectedfrom mobile devices 101. The radar signature or backscatter signal maybe modulated using a variety of different techniques and symbolconstellations including, BPSK, BASK, QAM or otherwise. As such,backscatter receive circuitry 250 includes the requisite filters,mixers, amplifiers, decoders, framers, and the like to demodulate/decodethe appropriate modulation scheme. Although FIG. 2 illustrates separatetransmit and receive antennas, in other embodiments, a singlebackscatter antenna may be used to both transmit EM energy 212 andreceive the backscatter signal 217. In other embodiment, multipletransmit and receive antennas may be used along with beam forming andtracking techniques.

Wireless interface(s) 240 represent one or more wireless communicationchannels that do not use backscatter communications. For example,wireless interface(s) 240 may be implemented using a WiFi transceiver, aBluetooth transceiver, an infrared transceiver, or otherstandardized/proprietary wireless communication systems. Wirelessinterface(s) 240 may facilitates non-backscatter communications withmobile devices 101 or with other devices. The wireless interface(s) 240may also provide a wireless network connection to the Internet or otherconsumer products (e.g., network attached storage, etc.) for basestation 203.

Wired interface(s) 230 may include any number of wired communicationports. For example, wired interfaces 230 may include an Ethernetcontroller, a universal serial bus (“USB”) port, or otherwise. TheEthernet controller may provide a network connection as well.

Power regulator 235 provides a wired power connection for powering theinternal components of base station 203. Since base station 203 is awired device, it is not constrained by a limited power budget likemobile devices 101. Backscatter communications leverage this asymmetricpower budget by pushing the power hungry generation of EM energy 212into base station 203 while mobile device 101 operate by reflecting EMenergy 212 (not independently generating EM radiation) generated at basestation 203.

Control circuitry 220 is the operational brains of base station 200. Itincludes logic 287 for coordinating the operation of the otherfunctional components and includes a processor and/or afield-programmable-gate-array (“FPGA”) for computational executions.Logic 287 may include hardware logic or software/firmware instructionsstored on one or more memory devices. For example, logic 287 may includeinstructions for establishing a wireless session with one or more mobiledevices 101, configuring and managing the wireless display sessions, andterminating the wireless display sessions.

Many commercial backscatter tags operate by encoding data using twodiscrete states. However, by using a larger number of states (increasingthe constellation of available communication symbols), quadratureamplitude modulation (“QAM”) can be achieved to deliver higher datarates in backscatter communication. For example, using sixteen states(e.g. 16-QAM) may result in a quadrupling of the data rate at thetradeoff of a lower SNR. Additionally, pairing the increase incommunication symbols with an increased data frequency has been shown toenable very high data rates (e.g. ˜100 Mbps). The higher data rates areusable for streaming data for large data application (e.g. cloud backup,video data).

To increase the constellation of available communication symbols beyondtwo data states (e.g. digital zero and one), the tag involved in thebackscatter communication must be able to generate the increased numberof communication symbols. Furthermore, generating the increased numberof communication symbols at higher frequencies allows for high-speedchannel having higher data rates than are available conventionally.However, to ensure that a tag is also able to communicate using theconventional low-speed schemes (having two data states), a tag thatcould communicate data using both the low-speed scheme and the disclosedhigh-speed data transmission would be advantageous. Accordingly, tagsdisclosed in the disclosure are configured to achieve high-speed datatransfer as well as be backward compatible to the conventional (e.g.“Gen 2”) lower-speed RFID protocol to ensure that the tag is able tocommunicate with existing base stations that utilize existing RFIDprotocols. Conventional lower-speed data may be sent at 125 kHz. whilethe higher-speed data may be sent at 25 MHz.

FIG. 3A illustrates a block diagram of an example device 310A thatincludes an example tag 307A, in accordance with an embodiment of thedisclosure. Device 310A may be one of the mobile devices described inFIG. 1. Example device 310A includes battery 325, processor 350, sensor375, first analog-to-digital converter (“ADC”) 361, a second ADC 363,and an antenna 345. Tag 307A includes modulation circuitry 330 andencoding module 320.

Antenna 345 is configured to receive electromagnetic radiation/energy(e.g. EM energy 212) from an antenna of a backscatter base station, suchas antenna 210 of base station 203. Antenna 345 may also be configuredto receive cellular data (e.g. 3G, 4G, LTE), WiFi (e.g. IEEE 802.11),and/or Bluetooth data. In other words, an existing antenna on a mobiledevice may be utilized for backscatter communication.

Modulation circuitry 330 is coupled to modulate antenna 345 between aplurality of impedance values (Z₀-Z₃₁ in FIG. 3A) applied to theantenna. Changing the impedance values of antenna 345 is one way ofchanging the vector radar cross-section of device 310A. Changing thevector radar cross-section changes the backscatter signal 217 thatantenna 345 reflects back to base station 203, which allows tag 307A tocommunicate data back to base station 203.

Modulation circuitry 330 includes A′ modulation circuitry 331, B′modulation circuitry 332, and 2-1 multiplexer (“MUX”) 339. A′ modulationcircuitry can only modulate the impedance of antenna 345 between a firstsubset of impedances values Z₀-Z₁₅. Impedances Z₀-Z₁₅ function tocommunicate sixteen symbols available for communicating with basestation 203 (e.g. to implement 16-QAM signaling). B′ modulationcircuitry can only modulate the impedance of antenna 345 between asecond subset of impedances values Z₁₆-Z₃₁. Impedances Z₁₆-Z₃₁ functionas sixteen corresponding symbols that communicate the same symbol asimpedances Z₀-Z₁₅. For example, in one embodiment, adjusting antenna 345to impedance Z₀ would communicate the same symbol to base station 203 asadjusting antenna 345 to impedance Z₁₆. In that example, Z₀ and Z₁₆ arecorresponding impedances that communicate the same correspondingsymbols. It is appreciated that the having 32 impedances values (Z₀-Z₃₁)that includes subsets of 16 impedance values is merely exemplary andthat more or less impedance values may be utilized in differentembodiments.

Encoding module 320 includes A′ selector logic 325 and B′ selector logic327. Encoding module 320 encodes first data 391 at a first higherfrequency (CLK1) by directing modulation circuitry 330 to select betweenthe plurality of impedances Z₀-Z₃₁. Encoding module 320 is also coupledto encode second data 392 at a second frequency (CLK2) by directingmodulation circuitry 330 to select among either a first subset Z₀-Z₁₅ ora second subset Z₁₆-Z₃₁ of the plurality of impedance values Z₀-Z₃₁.CLK2 operates at a lower frequency than CLK1.

In FIG. 3A, A′ selector logic 325 is coupled to receive first data 391and CLK1 operating at the first frequency. A′ selector logic 325 iscoupled to encode first data 391 at the first frequency by directing A′modulation circuitry 331 to select between Z₀-Z₁₅. B′ selector logic 327is also coupled to receive first data 391 and CLK1. B′ selector logic327 is coupled to encode first data 391 at the first frequency bydirecting B′ modulation circuitry 332 to select between Z₁₆-Z₃₁. MUX 339is coupled to receive data 392. The digital values of second data 392change corresponding to CLK2 which is at a lower frequency than thefrequency of first data 391 and CLK1. The digital values of data 392cause 2-1 MUX 339 to either couple the impedance values from A′modulation circuitry 331 or B′ modulation to antenna 345. Hence, theimpedance value applied to antenna 345 is limited to the first subset ofimpedance values of A′ modulation circuitry 331 when second data 392 hasa first state (e.g. digital zero) and the impedance value applied toantenna 345 is limited to the second subset of impedance values of B′modulation circuitry 332 when second data 392 has a second state (e.g.digital one).

A′ selector logic 325 and B′ selector logic 327 may be implemented usingmicrocontrollers, a logic array, discrete logic, or customApplication-Specific integrated circuit (“ASIC”). A′ modulationcircuitry 331 may be implemented with transistors T₀-T₁₅ that can beactivated to connect different impedance values Z₀-Z₁₅ to MUX 339 (andultimately antenna 345). Similarly, B′ modulation circuitry 332 may beimplemented with transistors T₁₆-T₃₁ that can be activated to connectdifferent impedance values Z₁₆-Z₃₁. Of course, other non-transistorswitches capable of switching at high frequencies can be used in placeof transistors. Those skilled in the art also appreciate thatalternative techniques and configurations for connecting differentimpedance values to antenna 345 may be implemented.

Attention is directed to FIG. 5 to further show how FIG. 3A communicatesboth a high-speed data stream and a lower-speed data stream viabackscatter techniques. FIG. 5 illustrates a chart showing vector radarcross sections (“RCSs”) over time of an antenna in a tag, in accordancewith an embodiment of the disclosure. It is appreciated that the vectorRCSs values in FIG. 5 are illustrative to convey an overall concept ofgiving different vector RCSs values to an antenna, but in operation thevector RCSs will include complex vector RCSs rather than strictly realvector RCS values. Furthermore, the impedance values applied to theantenna to give the antenna a given vector RCS value may be a compleximpedance value. FIG. 5 shows a first, second, third, and fourth period.Each period corresponds with the period associated with the frequency ofCLK2. The collection of vector RCS values in the first, second, third,and fourth periods are illustrated as groups 401, 402, 403, and 404,respectively.

In the first period, antenna 345 is modulated to different vector RCSvalues (by applying different impedance values to antenna 345, forexample) to encode first data 391 as different symbols that correspondwith the first subset of impedance values (e.g. Z₀-Z₁₅). In the secondperiod, antenna 345 is also modulated to different vector RCS values toencode first data 391 as different symbols that correspond with thesecond subset of impedance values. The first group of vector RCS values401 is close to central radar value 421 while the second group of vectorRCS values 402 is closer to central radar value 422. When the vector RCSvalues of antenna 345 are closer to central radar value 421 for a periodof CLK2, it conveys a first data state (e.g. digital zero) of seconddata 392 while vector RCS values closer to central radar value 422 for aperiod of CLK2 conveys a second data state (e.g. digital one) of seconddata 392. Thus, groups 401, 402, 403, and 404 convey that second data392 is zero-one-zero-zero in the first, second, third, and fourthperiods of FIG. 5. By modulating antenna 345 in this way, tag 307A cancommunicate first data 391 as high-speed data corresponding to CLK1 andalso communicate second data 392 as low-speed data corresponding to CLK2by switching between the first subset of impedance values and the secondsubset of impedance values to encode the second data. Since the tag mustswitch between the subsets of impedance values to communicate thelow-speed data, each symbol in the constellation of symbols beingcommunicated has a impedance value in the first subset that communicatesthat symbol and a corresponding impedance value in the second subsetthat also communicates that symbol so a particular high-speed symbol canbe communicated regardless of whether the first subset or the secondsubset of impedance values is being utilized. Of course, the changingimpedance values (to change the vector RCS of the antenna) are merely animplementation of modulating the backscatter signal by adjusting thein-phase and out-of-phase (i.e. I and Q quadrature) nature of thebackscatter signal that is reflected back to the base station.

A base station that is receiving the backscatter from tag 307A can applya filter (either analog or digital) with a cutoff frequency between thefirst frequency (CLK1) and the second frequency (CLK2) to isolate seconddata 392. Applying the filter will filter out the higher frequency data,but the combination of the higher frequency symbols will still comethrough as closer to a radar signal corresponding to central radar value421 or central radar value 422 to indicate two different discrete statesof lower-speed second data 392. It is understood that the word “central”in the term “central radar value” could be associated with a particularmapping (e.g. a specific region of a Smith Chart) from the backscattersignal to the signal received by the base station. Such mappings are notnecessarily linearly related to tag impedances, not necessarilyuniformly distributed, depend heavily on the RF link between tag andbase station, and are highly depended on the constellation of vector RCSvalues employed.

Returning to FIG. 3A, sensor 375 is coupled to provide a signal 371 toboth ADC1 361 and ADC2 362. Signal 371 may be a voltage, a current, orotherwise. Sensor 375 may be a biometric sensor that measures glucose orheart-rate, for example. ADC1 361 samples signal 371 at a firstfrequency associated with CLK1. ADC2 362 samples signal 371 at a secondfrequency associated with CLK2.

FIGS. 4A and 4B show that the first frequency (CLK1) is a higherfrequency than the second frequency (CLK2). FIG. 4A illustrates a chartshowing the voltage of a signal 471 over time and FIG. 4B illustrates azoomed-in portion of the chart in FIG. 4A, in accordance with anembodiment of the disclosure.

Signal 471 is an example of signal 371. Since ADC1 361 samples signal471 at a faster rate than ADC2 362 samples signal 471, first data 391has a higher resolution than second data 392. In one example, only oneADC is used to sample a signal at the first frequency (CLK1) and asubset of that signal sampled at the second frequency (CLK2) is sent assecond data 392. For example, an ADC could select every 10^(th) or every100^(th) sample to correspond to second data 392. Alternatively, thedata from one ADC could be processed to provide summary data as seconddata 392. One application for this implementation would be to sendelectrocardiography (“ECG”) heart signals where the waveform signal hasdiagnostically relevant details at the higher frequency, but the lowerfrequency could still indicate heartbeat in beats per minute. When tag307A simultaneously encodes first data 391 and second data 392 bychanging the impedance of antenna 345, it can encode both the higherresolution first data 391 and the lower resolution second data 392.Therefore, a base station configured to read the higher speed dataprotocol will be able to receive the higher resolution first data 391.However, if the base station is not configured to read the higher-speeddata protocol (a legacy base station), it will still be able to read thelower-speed data protocol and will be able to still receive thelower-speed data 392. The lower-speed data 392 protocol may be EPC Gen2backwards compatible so that legacy base stations can read thelower-speed data 392.

Processor 350 may also be coupled to send data to tag 307A forbackscatter communication. In one embodiment, processor 350 has accessto a memory and processor sends data from the memory to tag 307A to sendthe data to a base station. In one embodiment, medical records arestored in a memory and processor 350 facilitates streaming those medicalrecords to the base station using tag 307A. In one embodiment, processor350 is the main processor of device 310A. In one embodiment, first data391 and second data 392 include the same data content. Consequently,first data 391 is simply encoded faster than second data 392 and a basestation that is configured to receive the higher-speed protocol willreceive the data content faster on the high-speed channel. However, alegacy base station will still be able to receive the same data contentas second data 392 on the lower-speed channel. This configurationenables the updated base station to receive data faster from tag 307Awhile still enabling a non-updated base station to receive the samedata, albeit at a slower rate.

FIG. 3B illustrates a block diagram of a device 310B that includes anexample tag 307B, in accordance with an embodiment of the disclosure.Tag 307B shows a different hardware configuration of performing the samefunctions as described in association with tag 307A. Tag 307B includesencoding module 340 and modulation circuitry 351. Modulation circuitry351 is coupled to modulate antenna 345 between a plurality of impedancevalues Z₀-Z_(N). Encoding module 340 is coupled to encode first data ata first frequency by directing modulation circuitry 351 to selectbetween its impedance values Z₀-Z_(N). Encoding module 340 is alsocoupled to encode second data at a second frequency by directingmodulation circuitry 351 to select among either a first subset or asecond subset of the plurality of impedance values Z₀-Z_(N). Encodingmodule 340 includes selector logic 343. Selector logic 343 may beimplemented with a microprocessor or discrete logic. Selector logic 343receives first data 391 and second data 392. When second data 392 is afirst data state (e.g. digital zero), selector logic 343 limits itsselection of impedance values Z₀-Z_(N) to a first subset of impedancevalues. When second data 392 is a second data state (e.g. digital one),selector logic 343 limits its selection of impedance values Z₀-Z_(N) toa second subset of impedance values.

Device 310B includes sensor 377, sensor 379, first ADC 361, second ADC363. Sensor 377 provides signal 372 to first ADC 361 and sensor 379provides signal 373 to second ADC 363. Sensor 379 may be a heart-ratesensor, while sensor 377 may sense audio data (i.e. a microphone). Inone embodiment, the sensor that requires the lower data rates is coupledto generate second data 392 while the sensor that requires the higherdata rate is coupled to generate first data 391. It is understood thatthe different sensor configurations illustrated in FIGS. 3A and 3B maybe used in either Figure to generate first data 391 and second data 392.

In the examples illustrated in FIGS. 3A and 3B, the first subset ofimpedance values may include impedance values that are common to thesecond subset of impedance values. In a different embodiment, the firstsubset of impedance values may be not share impedance values with thesecond subset of impedance values.

After tag 307A or 307B encodes the data into backscatter signal 217, thebackscatter signal 217 is received by antenna 215 and decoded bybackscatter receiving circuitry 250. FIG. 2B is a functional blockdiagram illustrating backscatter receiving module 251 as one possibleexample of backscatter receiving circuitry 250, in accordance with anembodiment of the disclosure.

Backscatter receiving module 251 includes mixing block 252, front-endmodule 255, and decoding module 280. Front-end module 255 includeslow-speed filter 256 and high-speed filter 257. Decoding module 280includes a high-speed decoding module 281 having symbol translation unit282. Decoding module 280 also includes low-speed decoding module 283.Backscatter receiving module 251 will likely include additional analogand/or digital filters, ADCS, framing, and equalization modules andcircuitry that are not specifically illustrated as to not obscure theinvention.

Mixing block 252 is coupled to receive backscatter signal 217 fromreceiving antenna 215 and coupled to receive carrier frequency 253.Carrier frequency 253 will be the same as the frequency of the EM energy212 (the transmission signal) and mixing block 252 multipliesbackscatter signal 217 by carrier frequency 253 to isolate the modulatedportions of backscatter signal 217 that contain data. The modulatedportions of the backscatter signal continue on to front-end module 255.High-speed filter 257 and low-speed filter 256 both receive thebackscatter signal as they are configured in parallel in FIG. 2B. Thebackscatter signal may be optionally amplified prior to reaching thefilters.

Low-speed filter 256 is configured to isolate the low-speed data fromthe high-speed data. If the high-speed data is encoded by a tag (e.g.307A or 307B) into backscatter signal 217 at a first frequency (e.g. 25MHz.) and low-speed data is encoded at a second frequency (e.g. 125kHz.), low-speed filter 256 isolates the low-speed data by filtering outfrequencies that are above 125 kHz. For example, low-speed filter 256may be a low pass filter with a cutoff frequency between the firstfrequency and the second frequency. Therefore, low-speed filter 256passes low-speed data 259 in response to the backscatter signal. Since,low-speed filter 256 filters out the high-speed transitions of thehigh-speed data, the values of the high-speed symbols are effectivelyaveraged. This filtering determines whether the high-speed symbols arefrom a first subset of the symbols or a second subset of the symbols ata given time period because the value of the low-speed data willindicate which subset of symbols was utilized during the given timeperiod. When the high-speed symbols are being received in the firstsubset (for a certain length of time), it indicates a first state (e.g.zero) of the low-speed data and when the high-speed symbols are beingreceived in the second subset (for the certain length of time), itindicates a second state (e.g. digital one) of the low-speed data. Thelength of time that the high-speed symbols need to stay in a subset toindicate a certain data state corresponds with a period of the secondfrequency (e.g. 125 kHz.).

The high-speed symbols include a first subset of symbols and a secondsubset of symbols. The first subset of symbols may correspond to thefirst subset of impedance values described in FIGS. 3A and 3B.Similarly, the second subset of symbols may correspond to the secondsubset of impedance values described in FIGS. 3A and 3B. Each symbol inthe first subset has a corresponding symbol in the second subset so thattag 307 can communicate the full constellation of symbols whether or notit is utilizing the first subset of vector radar cross-sections of theantenna (which are generated by the first subset of impedances) or thesecond subset of vector radar cross-sections of the antenna (which aregenerated by the second subset of impedances). The corresponding symbolscommunicate the same data value even though different symbols are used.In one embodiment, Z₀ corresponds with Z₁₆, Z₁ corresponds with Z₁₇ . .. and Z₁₅ corresponds with Z₃₁. So in that embodiment, giving antenna345 either Z₀ or Z₁₆ communicates the same symbol or data character.

High-speed filter 257 is configured to pass high-speed data 258 inresponse to receiving the backscatter signal. High-speed filter 257 mayinclude a bandpass filter to isolate the high-speed data. Either or bothof high-speed filter 257 and low-speed filter 256 may be implemented bya processor configured as a software-defined-radio.

High-speed decoding module 281 receives high-speed data 258 andlow-speed decoding module 283 receives low-speed data 259. High-speeddecoding module 281 is configured to generate first data 271 by decodingthe high-speed symbols encoded at a first frequency (e.g. 25 MHz.). Tag307 may have encoded the high-speed symbols at the first frequency(CLK1) by modulating the impedance of antenna 345. If tag 307 isutilizing QAM, high-speed decoding module 281 will include a QAMdecoding unit. Low-speed decoding module 283 is configured to outputsecond data 272 in response to low-speed data 259, which was encoded ata second frequency (e.g. 125 kHz.). Tag 307 may have encoded thelow-speed data at the second frequency (CLK2) by driving MUX 339 tolimit the impedance values of antenna 345 to the first or second subsetof impedance values corresponding to circuitry 331 or 332.

High-speed decoding module 281 includes 2-1 symbol translation unit 282in FIG. 2B. Since the high-speed symbols include a first subset and asecond subset of symbols and a given symbol in the first subset has acorresponding symbol in the second subset that communicates the samesymbol or data character, symbol translation unit 282 outputs the properdata character in response to receiving either of the correspondingsymbols. Decoding module 280 may also include a checksum to ensureaccurate data reception.

FIG. 6 illustrates a flow chart illustrating a process of tag-sidebackscatter communication, in accordance with an embodiment of thedisclosure. The order in which some or all of the process blocks appearin process 600 should not be deemed limiting. Rather, one of ordinaryskill in the art having the benefit of the present disclosure willunderstand that some of the process blocks may be executed in a varietyof orders not illustrated, or even in parallel.

In process block 605, electromagnetic energy (e.g. EM energy 212) isreceived from a base station. First data is encoded (at a firstfrequency) by adjusting a radar cross-section of a device between aplurality of radar cross-sections, in process block 610. In the examplesillustrated in FIGS. 3A and 3B, the vector radar cross-section ofantenna 345 is adjusted by modulating the impedance of the antennabetween a plurality of impedance values. However, there are additionalways of adjusting a radar cross-section of a device. For example,micro-electro-mechanical systems (“MEMS”) can be manipulated to changethe radar cross-section of a device. For example, MEMS that adjust atilt of a radar reflecting material (e.g. metal) also changes the radarcross-section of a device. Furthermore, multiple MEMS could bemanipulated in patterns (in addition to tilt control) to generatedifferent radar cross-sections. Another way to adjust the radarcross-section of device includes infusing metallics into liquid crystaland controlling the alignment of the metallics using the liquid crystaland in turn changing the radar cross-section of a device. Here again, anarray of independently selectable liquid crystals having infusedmetallics would also create additional adjustability of the radarcross-section to generate a plurality of different radar cross-sections.Additional ways of adjusting a radar cross section includes varactors,PIN diodes, variable attenuators, and variable phase shifters.

In process block 615, second data is encoded (at a second frequency thatis lower than the first frequency) by limiting the adjusting of theplurality of radar cross-section for a period of time. The period oftime corresponds with a period of the second frequency. Process 600continues to encode first data and second data, as needed to communicatethe required amount of data to a base station.

FIG. 7 illustrates a flow chart illustrating a method of backscattercommunication utilizing a base station, in accordance with an embodimentof the disclosure. The order in which some or all of the process blocksappear in process 700 should not be deemed limiting. Rather, one ofordinary skill in the art having the benefit of the present disclosurewill understand that some of the process blocks may be executed in avariety of orders not illustrated, or even in parallel.

In process block 705, a backscatter signal (e.g. backscatter signal 217)is received with a backscatter receiving antenna (e.g. antenna 215). Thebackscatter signal is a modulated version of an electromagnetictransmission signal that is transmitted by a transmitting antenna (e.g.antenna 210). The backscatter receiving antenna and the transmissionantenna may be the same antenna, in some embodiments. The transmissionsignal may be reflected back as a modulated backscatter signal by a tag(e.g. tag 307) within a mobile device (e.g. device 310).

High-speed symbols encoded into the backscatter signal are decoded inprocess block 710. The high-speed symbols are encoded into thebackscatter signal at a first frequency (e.g. 25 MHz.)

In process block 715, the high-speed symbols are translated into firstdata (e.g. data 271). The high-speed symbols include a first subset ofsymbols and a second subset of symbols. Corresponding symbols from thefirst subset and the second subset are translated to have the same datacharacter in the first data.

Second data (e.g. data 272) is decoded in response to low-speed dataencoded into the backscatter signal, in process block 720. The low-speeddata is encoded into the backscatter signal at a second frequency (e.g.125 kHz.) that is less than the first frequency that the high-speedsymbols are encoded at. Since tag 307A encodes the second data bysending a first subset of high-speed symbols for a length of time (e.g.between signals of CLK2) to indicate a first state (e.g. digital zero)of the second data and by sending a second subset of high-speed symbolsto indicate a second state (e.g. digital one) of the second data, thesecond data gets a first state when the high-speed symbols are in thefirst subset for a certain length of time and second data gets a secondstate when the high-speed symbols are in the second subset for thelength of time. The length of time corresponds with a period of thesecond frequency, in one embodiment.

It is understood that process blocks 710/715 and 720 may be executed inparallel (as illustrated) or that they may be executed serially. Process700 may repeat to continually decode data coming from backscattersignals. It is appreciated that a base station that utilizes process 700will be capable of decoding high-speed data as well as supportinglow-speed data. Therefore, a tag that sends only low-speed data will becompatible with the base station and a tag that sends only high-speeddata (utilizing QAM, as an example) will also be compatible with thebase station. Furthermore, a tag (e.g. tag 307) that can send bothlow-speed data and high-speed data will also be compatible with a basestation (e.g. base station 203) utilizing process 700.

The processes explained above are described in terms of computersoftware and hardware. The techniques described may constitutemachine-executable instructions embodied within a tangible ornon-transitory machine (e.g., computer) readable storage medium, thatwhen executed by a machine will cause the machine to perform theoperations described. Additionally, the processes may be embodied withinhardware, such as an application specific integrated circuit (“ASIC”) orotherwise.

A tangible non-transitory machine-readable storage medium includes anymechanism that provides (i.e., stores) information in a form accessibleby a machine (e.g., a computer, network device, personal digitalassistant, manufacturing tool, any device with a set of one or moreprocessors, etc.). For example, a machine-readable storage mediumincludes recordable/non-recordable media (e.g., read only memory (ROM),random access memory (RAM), magnetic disk storage media, optical storagemedia, flash memory devices, etc.).

The above description of illustrated embodiments of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific embodiments of, and examples for, the invention aredescribed herein for illustrative purposes, various modifications arepossible within the scope of the invention, as those skilled in therelevant art will recognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific embodimentsdisclosed in the specification. Rather, the scope of the invention is tobe determined entirely by the following claims, which are to beconstrued in accordance with established doctrines of claiminterpretation.

What is claimed is:
 1. A mobile device comprising: an antenna configuredto receive electromagnetic energy from an RFID base station; modulationcircuitry coupled to modulate the antenna between a plurality ofimpedance values to change a radar cross-section of the mobile devicefor communicating with the RFID base station via backscattering of theelectromagnetic energy; and an encoding module coupled to encode firstdata at a higher frequency by directing the modulation circuitry toselect between the plurality of impedance values and further coupled toencode second data at a lower frequency by directing the modulationcircuitry to select among either a first subset or a second subset ofthe plurality of impedance values for a length of time, wherein a firstdata state of the second data is represented by selecting among thefirst subset during the length of time, and wherein a second data stateof the second data is represented by selecting among the second subsetduring the length of time.
 2. The mobile device of claim 1 furthercomprising: a biometric sensor coupled to generate at least one of thefirst data or the second data.
 3. The mobile device of claim 1, whereinthe antenna is also configured to receive at least one of WiFi orcellular data.
 4. The mobile device of claim 1, wherein the first datais a higher resolution version of the second data.
 5. The mobile deviceof claim 4 further comprising: a first analog-to-digital converter(“ADC”) configured to sample a first signal at the higher frequency togenerate the first data; and a second ADC configured to sample a secondsignal at the lower frequency to generate the second data.
 6. The mobiledevice of claim 1, wherein the first subset of the plurality ofimpedance values includes impedance values that are common to the secondsubset of impedance values and also includes impedance values that aredifferent from the second subset of impedance values.
 7. The mobiledevice of claim 1, wherein the encoding module includes first selectorlogic coupled to select impedance values from among the first subset inresponse to the first data, and wherein the encoding module includessecond selector logic coupled to select impedance values from among thesecond subset in response to the first data, the encoding moduleconfigured to connect the antenna to the impedance values from the firstsubset when the second data has the first data state and the encodingmodule configured to connect the antenna to the impedance values fromthe second subset when the second data has the second data state.
 8. Themobile device of claim 1, wherein the first subset of the plurality ofimpedance values does not share impedance values with the second subsetof the plurality of impedance values.
 9. The mobile device of claim 1,wherein the mobile device comprises one of a mobile phone, a headwearable display, a wrist watch, a body-mountable device, a bodyimplantable device, a tablet, or a laptop.
 10. The mobile device ofclaim 1, wherein the length of time corresponds to a period of the lowerfrequency.
 11. A radio-frequency identification (“RFID”) tag comprising:modulation circuitry coupled to modulate an antenna between a pluralityof impedance values to change a radar cross-section of the RFID tag forcommunicating with an RFID base station, wherein the antenna isconfigured to receive electromagnetic energy from the RFID base station;and an encoding module coupled to encode first data at a higherfrequency by directing the modulation circuitry to select between theplurality of impedance values and further coupled to encode second dataat a lower frequency by directing the modulation circuitry to selectamong either a first subset or a second subset of the plurality ofimpedance values for a length of a time, wherein a first data state ofthe second data is represented by selecting among the first subsetduring the length of time, and wherein a second data state of the seconddata is represented by selecting among the second subset during thelength of time.
 12. The RFID tag of claim 11, wherein the encodingmodule includes first selector logic coupled to select impedance valuesfrom among the first subset in response to the first data, and whereinthe encoding module includes second selector logic coupled to selectimpedance values from among the second subset in response to the firstdata, the encoding module configured to connect the antenna to theimpedance values from the first subset when the second data has thefirst data state and the encoding module configured to connect theantenna to the impedance values from the second subset when the seconddata has the second data state.
 13. The RFID tag of claim 11, whereinthe first subset of the plurality of impedance values includes impedancevalues that are common to the second subset of impedance values and alsoincludes impedance values that are different from the second subset ofimpedance values.
 14. The RFID tag of claim 11, wherein the first subsetof the plurality of impedance values does not share impedance valueswith the second subset of the plurality of impedance values.
 15. TheRFID tag of claim 11, wherein the first data is a higher resolutionversion of the second data.
 16. The RFID tag of claim 11, wherein theantenna is also configured to receive at least one of WiFi, Bluetooth,or cellular data.
 17. A method of backscatter communication comprising:receiving electromagnetic energy from a base station; encoding firstdata by adjusting, at a first frequency, a radar cross-section of adevice between a plurality of radar cross-sections of the device tomodulate the electromagnetic energy reflected back to the base station;and encoding second data at a second frequency by limiting the adjustingof the plurality of radar cross-sections to either a first subset or asecond subset of the plurality of radar cross-sections for a length oftime corresponding to one period the second frequency, wherein a firstdata state of the second data is represented by selecting among thefirst subset during the length of time, and wherein a second data stateof the second data is represented by selecting among the second subsetduring the length of time, the second frequency being less than thefirst frequency.
 18. The method of claim 17, wherein the first subset ofthe plurality of radar cross-sections includes radar cross-sections thatare common to the second subset of radar cross-sections and alsoincludes radar cross-sections that are different from the second subsetof radar cross-sections.
 19. The method of claim 17, wherein the firstsubset of the plurality of radar cross-sections does not share radarcross-sections with the second subset of the plurality of radarcross-sections.
 20. The method of claim 17, wherein adjusting the radarcross-section of the device includes selecting different impedancevalues of an antenna of the device.
 21. The method of claim 17, whereinthe second data is derived from the first data or is a smaller subset ofthe first data.
 22. The method of claim 17, wherein the second data isbackwards compatible with EPC Gen2 RFID protocol.